Hard wired irrigation systems are based on a 24 volts AC RMS (VAC) supply and use a common to the valves plus one control wire to each solenoid. Frequently, several solenoid valves are required to be operated simultaneously. In large turf and agricultural applications, the length of the runs of wire from the controller to the solenoid can be as long as 15,000 feet (round trip). All solenoids require a minimum level of voltage and current for proper operation. A 24 Volt AC (VAC) solenoid can require typically 20 VAC (volts AC) at 0.45 amps inrush current for small valves (3 inch and under) and as much as 1.5 amps for larger valves. The problem is that these current loads cause a voltage drop between the controller and the solenoids. This is calculated by the equation: EQU V.sub.D =I.times.R.times.L
V.sub.D =voltage drop in volts PA1 I=current load in amps PA1 R=resistance factor (ohm/1000 ft.) PA1 L=length of wire in thousands of feet PA1 For 14 gauge solid copper wire, R=2.5 ohms/1000 ft. PA1 For 12 gauge solid copper wire, R=1.588 ohms/1000 ft. PA1 For 10 gauge solid copper wire, R=1.0 ohms/1000 ft. PA1 For 8 gauge solid copper wire, R=0.628 ohms/1000 ft. PA1 For 6 gauge solid copper wire, R=0.395 ohms/1000 ft. PA1 1. Eliminate the inrush current demanded from the source by AC operated solenoids. PA1 2. Once the solenoid has been actuated, to keep it energized with a lower level of voltage and current.
For an example, an inrush load of one amp at 5000 feet using 14 gauge wire should cause a voltage drop of: EQU V.sub.D =1amp.times.2.5.times.5=12.5 volts.
Typically, there is a 24 VAC supply at the controller. By the time the solenoid is reached, 24-12.5 volts=11.5 volts is available. Normally about a minimum of 20 VAC is required for reliable operation.
Going to 12 gauge would give us a drop of: V.sub.D =1.times.1.588.times.5=7.94 volts. This would be about 16 volts, still not enough.
Going to 10 gauge would give us a drop of: V.sub.D =1.times.1.times.5=5 volts. Since this is still less than 20 VAC, 8 gauge would be required.
For a cost analysis, 14 gauge costs about $28.00 per 1000 feet. 8 gauge wire costs about $135 per 1000 feet. A net cost difference of $107.00 per 1000 feet.times.5=$535 per value. If there are 20 valves on this job, several thousand dollars of wire would be required. In addition, handling of the heavier 8 gauge is more difficult than direct burial 14 gauge. With the module, three basic problems are overcome: high cost, difficult installation, and high energy requirements.